Overheating when A/C on?

The car runs at a normal temperature without A/C on holds steady around 80-90 degrees Celsius. Once I turn the A/C on, after about 5 minutes the temperature starts to rise and will continue to rise until the car says stop driving high coolant temperature. It does not run hot if I am on the highway with the A/C on and holding a steady speed around 70 MPH but once I start stopping at traffic lights it starts to overheat. I changed the thermostat it didn’t do any good. Can you tell me what might be my problem?